# algorithym to calculate MAH on a battery charger

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Greetings,

I'm trying to write a routine to calculate milliampere-hours (MAH) for
a battery charger I'm designing.  The approach I'm intending to use is
to sample the charging current once every second, store the first
sample in a 32 bit accumulator and add all following samples to it
while keeping track of the number of samples.  Every ten seconds or
so, I'll divide that sum by the number of samples to get the average
current.  If I then multiply that by the percentage of an hour that
has passed I should think that would be the current MAH that has been
input into the battery.  Total charge time will only be a couple of
hours.

Does anyone see anything wrong with this approach?

Thanks for you help.

Re: algorithym to calculate MAH on a battery charger

I think that'd be doing it the wrong way round.  You shouldn't ever
have to "sample" the charge current in an MCU-controlled battery
charger.  That current is supposed to be the primary parameter your
MCU is in control of.  As such, your program should already know by
itself each time that current is changed.  So log those events, with a
timestamp from your timer you keep running in the background.  The
maths stay the same.

Unnecessary.  Just add them up.  If you sample each second, each
milli-amp you sampled counts as one milliamp-second.  You only have to
divide at the end, by 3600 seconds/hour, to adapt the units.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: algorithym to calculate MAH on a battery charger

You would have to measure current as part of the control loop.
The milliamp/seconds is what I use.

Re: algorithym to calculate MAH on a battery charger

<snip>

I meant to reply to that in my other update post.  The charger is
designed for Lithium Polymer batteries and the charger switches  from
constant current to constant voltage at 4.2volts and the current
begins to drop from that point on so I do have to sample it
periodically.

Thanks again,
Art

Re: algorithym to calculate MAH on a battery charger
On 10 Jun 2004 21:34:42 GMT, Hans-Bernhard Broeker

Thanks, Hans.  I hadn't thought of it in those terms.  MASecs works
just fine.

Anyone else have ideas?

Art

Re: algorithym to calculate MAH on a battery charger
On Fri, 11 Jun 2004 13:57:27 GMT, Art K6KFH

What if you don't know the current ?

You might want a running answer and not have to wait an hour or 2....

Re: algorithym to calculate MAH on a battery charger

[...]

Then you measure it.  What I was trying to point out there that
there's a significant probability that there's no need to measure,
because the MCU will have told a current source what current to supply
to the cell being charged --- that's how charging tends to work: you
set a current, and watch the voltage, rather than the other way round.

Then you divide each time you need an update of the running answer.
Or just get used to getting answers in units of milliamp-seconds,
instead of the more conventional mAh.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: algorithym to calculate MAH on a battery charger
says...

Or just divide by 3600 :)  Since you are counting mA-seconds the amount
of time the count runs for isn't relevant.  It's just a change in the
unit of measure. 3.6A for 1 sec is the same as 1 mA for 3600 seconds.
Whether the battery can take that kind of charge is another question.

Robert

Re: algorithym to calculate MAH on a battery charger
In comp.arch.embedded,

That depends on the battery type, which I don't see mentioned in this
post (was it somewhere in the thread? then I missed it). You are right
for NiMh/Cd. But for Lead Acid batteries, you set the voltage (and a
current limit) and observe the current.

--
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

Magic is always the best solution -- especially reliable magic.

Re: algorithym to calculate MAH on a battery charger
On Wed, 30 Jun 2004 15:22:47 +0200,

I guess I should have clarified myself.  I'm designing a Lithium Poly
battery charger.  This requires a constant current until the cell
reaches 4.2 volts at which time it changes to  a constant voltage of
4.2 volts and the current begins decreasing until it drops to some
arbitrary value.  During this time the current isn't controlled by the
charger and so it has to be measured periodically.  It spends a lot of
time in this last part of the charge.   Thanks to previous posters I
have resolved the problem.  Thanks for all your responses.

Regards,

Art